1. Field of the Invention
The present invention relates to distributed Bragg reflectors and to organic electroluminescent light emitting devices (hereinafter referred to as OELDs). In one aspect, the invention is concerned with improvement of the output characteristics of OELDs.
2. Description of the Related Art
A schematic illustration of the output spectrum of a conventional OELD is shown in FIG. 1, from which it will be appreciated that the spectrum has a long tail towards higher wavelengths. The output spectrum illustrated in FIG. 1 is undesirable because pure colours can not be obtained.
The structure of an OELD is shown in FIG. 2. The structure comprises, in sequence, a transparent substrate 10 (for example, formed of glass), a distributed Bragg reflector (hereinafter referred to as a DBR) 12, a transparent electrode 14 (an anode, for example formed of Indium Tin Oxide), an organic electroluminescent light emitting layer 16, and a cathode 18. The cathode 18 and DBR 12 act as mirrors and the structure functions as a microcavity, in much the same way as the conventional laser structure. Spontaneous emission from the active material within the cavity is enhanced at the mode (or design) wavelength of the cavity and suppressed elsewhere. The desire is to gain sufficient control to enable the practical implementation of multicolour organic light emitting devices. In order to implement a full colour device, the DBR must have high reflectivity over the whole visible spectrum.
The DBR consists of a stack of inorganic dielectric films, with alternate layers of two materials having a difference in refractive index therebetween. For example, one layer may be formed of Titanium dioxide (TiO2), with a refractive index of approximately 2, and the other may be formed of Silicon dioxide (SiO2), with a refractive index of approximately 1.5. The materials are selected so as to maximise reflectivity and minimise absorption. The smaller the difference between the refractive index of the two materials, the sharper is the selectivity of the wavelength of the reflected light. That is, the width of the stop band is approximately xcexxcex94n/n where xcex94n is the index difference between the layers that constitute the DBR, xcex is the centre wavelength of the stop band and n is the average refractive index. Thus it will be apparent that the materials just mentioned can not cover the whole visible spectrum, as required.
An arrangement of the type described above is disclosed in an article by Tetsuo Tsutui et al, Applied Physics Letters 65 (15) p.1868, 10th, Oct. 1994.
Formation of a DBR of the described type is by way of deposition processes. These processes are very time consuming and significantly more difficult than, for example, the deposition of metal films. The dioxide materials used for the DBR deposit on the walls of the fabrication chamber and flake off easily, thus causing contamination. The structure described with reference to FIG. 2 is thus not practicable for commercial fabrication and is too troublesome to produce a DBR which varies across a substrate so as to provide different wavelength selectivity across different portions of the substrate.
According to a first aspect of the present invention there is provided a distributed Bragg reflector comprising a stack of alternate layers of a first material and a second material wherein the first and second materials are both organic materials.
According to a second aspect of the present invention there is provided an organic electroluminescent light emitting element comprising: a transparent substrate, a transparent electrode formed on the substrate, a distributed Bragg reflector formed on the transparent electrode, an organic electroluminescent light emitting material formed on the distributed Bragg reflector and an electrode formed on the light emitting material, the distributed Bragg reflector being in accordance with the first aspect of the invention.
According to a third aspect of the present invention there is provided a multicolour light emitting device comprising a plurality of light emitting elements according to the second aspect of the invention.
According to a fourth aspect of the present invention there is provided a method of manufacturing a distributed Bragg reflector comprising the steps of forming a stack of alternate layers of a first organic material and a second organic material using inkjet technology.
According to a fifth aspect of the present invention there is provided a method of manufacturing an organic electroluminescent light emitting element comprising the steps of: providing a transparent substrate, forming a transparent electrode formed on the substrate, forming a distributed Bragg reflector on the transparent electrode using the method of the fourth aspect of the invention, forming an organic electroluminescent light emitting material on the distributed Bragg reflector, and forming an electrode on the light emitting material.
According to a sixth aspect of the present invention there is provided a method of manufacturing a multicolour light emitting device comprising the steps of: forming a plurality of light emitting elements using the method of the fifth aspect of the invention and therein forming the first and second materials in different thickness layers to provide different mode wavelength areas on the substrate.